The present invention relates to a reversal development method for providing reversal toner images by causing magnetic toners to adhere to the uncharged areas of a latent electrostatic image formed on an image-carrying member surface.
Encouraged by wide use of computers in recent years, developments have been made actively on printers as peripheral equipment for producing hard copies as the outputs of information in the form of letters and figures. The printers which were most commonly used conventionally are so-called impact-type printers which produce prints by physically impinging selected printing types upon papers via ink ribbons. However, the appearance of higher-performance computers and the diversification of information to be processed have necessitated the quick processing of large amounts of information and various types of output forms such as various sizes of letters such as Chinese characters and figures. These requirements cannot be met by the conventional impact-type printers, so efforts have been made to develop new non-impact-type printers (electronic printers).
The nonimpact-type printers are classified into three groups; an electrophotographic type, an electrostatic type and a ink jet type from the viewpoint of recording methods. In order to cope with the recent trends of increase in recording speeds and densities, the electrophotographic type appears to be the most promising.
The principles of recording by electrophotographic printers are essentially the same as those of usual copiers: The recording process comprises the steps of uniformly charging a photosensitive member surface, forming a latent electrostatic image by exposure, developing the latent electrostatic image with a toner, transferring the toner image onto a plain paper and fixing. Since in the electrophotographic printer, information supplied from a computer is written on a uniformly charged photosensitive surface with a laser beam, etc., and toner is caused to attach to the written areas, namely the exposed areas of the surface, development should be done in reverse.
Dry developers for the reversal development are usually two component-type developers which consist of magnetic carriers and non-magnetic toners like those for copiers. Most printers now in use utilize such developers.
When the two component-type developers are used, the toners have enough electrostatic charges due to the triboelectrification with the carriers so that they can produce exact development of the nonimage areas of the latent electrostatic image. Further, since the toners retain electrostatic charges after the development, the toner image can be electrostatically transferred onto a commonly available plain paper, resulting in high-quality print image. The use of the two component-type developers, however, requires means for keeping carrier-toner mixtures at constant mixing ratios to maintain the constant optical densities of the resulting images, resulting in larger and more complicated developing apparatuses. In addition, the mixing and stirring of the carriers with the toners for extended periods of time leads to the formation of toner layers on the carrier surfaces, deteriorating the triboelectric characteristics of the carriers, which requires the periodic replacement of the carriers.
To solve these problems, one component-type developers consisting only of magnetic toner particles as dry developing components for developing latent electrostatic images have been developed and put into practical use.
In a reversal development method using the so-called magnetic toners, the toners are generally attracted to the nonimage areas by applying DC bias voltage of the same polarity as that of the latent electrostatic image to a conductive sleeve holding the magnetic toners charged with the same polarity as that of the electrostatic image. In order that the magnetic toners can be charged with the predetermined polarity, they should be a chargeable type containing charge controlling agents in the inner and/or the surface thereof. See Japanese Patent Laid-Open No. 55-48754. Such chargeable magnetic toners are caused to have charges by contact with a sleeve or a doctor blade or with each other.
In the electrostatic transfer of the toner image produced by the development onto a transfer sheet, it is common to use insulating magnetic toners having high electric resistance in order to prevent the deterioration of the transferred image. See, for instance, U.S. Pat. No. 4,121,931.
However, when the insulating magnetic toners of the above-mentioned chargeable type are charged with the same polarity as that of the latent electrostatic image to conduct reversal development, it is actually inevitable that they are inferior to the two component-type developers in terms of image qualities. Specifically, when the magnetic toners adapted to be charged with the same polarity as that of the latent electrostatic image are used, they can provide image densities on the same level as those of the two component-type developers, but they fail to give sufficient resolution, and images developed therewith tend to have dust consisting of dispersed toner particles adhered around their peripheries.